1. Field of Invention
The present invention relates to measuring instruments and more particularly, to an improved instrument for measuring optical radiation power which is capable of making absolute or relative measurements and for providing results either with a visible digital display or with analog or digital signals suitable for application to other instruments or systems.
In recent years, greater use has been made of fiber optics for high speed transmission of information using light as the signal carrier. Optical radiation at the frequencies normally employed utilizes sources whose wavelengths range from under 400 nanometers through 1,200 nanometers in typical applications. These high frequencies permit information rates substantially greater than can be employed using the normal electromagnetic spectrum ranging through microwaves.
Accordingly, the need has arisen for better instruments that can be employed to measure the relatively low levels of light energy such as are used in fiber optics communications. Further, it useful to be able to measure all aspects of a fiber optic system, including light sources and emitters, photoreceivers, the transmission of fiber cables, and the losses that can be encountered through connectors or splices.
2. Description of the Prior Art
The instruments available to measure light intensity have classically been linear amplifying devices such as radiometers, photometers of power meters. These instruments most often functioned as linear current measuring instruments, i.e., recording and displaying the currents from photosensors. As light levels change, it was necessary to change the scale setting in such instruments. Their output was in units of optical power, such as watts, mwatt, .mu.watt, nwatt, picowatt.
These prior art instruments have also been used for measuring light power in fiber optic systems. However, the most common unit for fiber optic measurements is not based on the watt or milliwatt, but the dBm (decibel milliwatt). Therefore, the measurements provided by linearly responding, prior art, power meters were not immediately useful. A mathematical conversion to a decibel reading was invariably necessary. In addition, the signal levels from fiber optic systems can vary over at least nine decades. Thus the scale settings of these linear reading prior art instruments might be required to be changed up to nine times during a measurement cycle. Further, such instruments measure only one signal at a time, and fiber optic systems often require that two signals be simultaneously measured, as from a fiber optic "T"-coupler or "star" coupler. For these measurements with prior art instrumentation, two instruments were required.
3. Summary of Invention
According to the present invention, all of these deficiencies of prior art fiber optic power measuring instruments have been overcome. The instrument of the present invention reads out directly in decibel units. It requires no range switching, yet covers a nine decade measurement range (90 dB), from 1 picowatt (-90 dBm) to 1 milliwatt (0 dBm) of optical power. It can measure one optical signal power level in absolute units (dBm or dB.mu.), or measure and compare two signal power levels (dB) simultaneously.
As noted above, optical radiation power is generally measured in units of watts, but the art has adopted the convention of expressing power in decibels (dB) according to the following formula: EQU dB=10 log (P.sub.sig /P.sub.ref) (Equation 1)
In Equation 1, P.sub.sig is the power to be measured and P.sub.ref represents the power of a known source. If the reference source is one milliwatt, then Equation 1 becomes: EQU dBm=10 log (P.sub.sig /1 mw) (Equation 2)
For even lower levels of power, the reference source might be as small as one microwatt, resulting in the following equation: EQU dBm.mu.=10 log (P.sub.sig /1 .mu.w) (Equation 3)
Of course, if a reference is not available and, instead, both signals are unknown, then the power measured would be the log ratio of the two input signals, expressed in decibels (dB).
By utilizing an internal calibrated current reference source corresponding either to one milliwatt or one microwatt, an unknown source of optical power can be measured and expressed in terms of absolute units of either decibel milliwatts (dBm) or decibel microwatts (dB.mu.). When both channels are utilized, then the optical power applied to the two channels is compared and a readout in "relative" units of decibels (dB) can be displayed.
Should it be necessary to express a decibel measurement in watts, the following equation can be derived from Equation 1: EQU P.sub.sig /P.sub.ref =10 .sup.dB/10 (Equation 4)
The instrument of the present invention is also provided with a sample-hold circuit which enables a subsequent measurement to be compared to an earlier measurement. The difference can be displayed in relative units of decibels. This would be useful in, for example, determining the loss attributable to a coupling. The transmitted optical power of a fiber optic line is measured first with and then without the coupling, and the difference displayed.
In prior art systems, as described above, the output of a photodetector is generally applied to a preamplifier which converts AC to DC as part of a feedback loop. A separate, second stage linear amplifier which includes temperature compensation resistors, is then employed.
If a second, substantially identical signal channel is used, differential measurements can be made if both are connected to a differential amplifier. The resultant output signal can then be processed to provide a display.
According to the present invention, a logarithmic amplifier is utilized which includes a non-linear element such as a transistor in the feedback loop. A direct readout in the desired dB units is then achieved. Further, the conventional approach, which uses a second stage linear amplifier with temperature compensating resistors cannot be employed, since the log of the average of the signal is not equal to the average of the log. Only by using conventional amplifiers and substantially linear feedback elements is the log of the average equal to the average of the log.
According to the present invention, a special resistive element with a positive temperature coefficient is utilized in a differential mode for temperature compensation. The output signal of the logarithmic amplifier is applied to an analog-to-digital converter which, in turn, drives the digital display. The difference between the output of a first channel and the output of a second, identical channel is then divided by the temperature compensating voltage. This, in effect, compensates both channels. The transistors in the feedback loops of the logarithmic amplifiers must be suitably matched. The temperature compensating resistor is then selected to have the identical temperature coefficient.
The use of this simple temperature compensation circuit facilitates the utilization of an analog to digital converter for a direct digital readout. Without temperature compensation, an inaccurate reading would result.
Absolute light level readings can be taken by generating an internal electrical current that is applied to one of the channels as though it were a photodetector output. This internal reference will then be temperature compensated to the same extent as the incoming signal, and will be treated as though it were a light input of known magnitude. Alternatively, a precisely calibrated light source of known intensity can be applied to the reference channel and compared to the unknown signal.
A power source is also provided to energize a standard optical source of known intensity for calibrating photodetectors whose response characteristics have not been accurately determined.
The use of logarithmic amplifiers also enables a single instrument to operate over a substantially greater range of intensities. Signal levels from one picowatt have been measured, representing -90 dBm. In the preferred embodiment, nine decades of intensity can be accommodated with either milliwatt or microwatt measurements and 0.1 dB resolution. Alternatively, four decades can be measured if 0.01 dB resolution is desired.
Other novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.